1. Field of the Invention
The present invention relates generally to spread spectrum receivers, and in particular to methods of optimizing the equalization of a spread spectrum signal transmitted through multiple resolvable fading paths channel. The present invention is suitable for use in applications involving W-CDMA transmission techniques, and it will be convenient to describe the invention in relation to that exemplary application.
2. Description of the Related Art
In W-CDMA communication systems, multi code signals at the transmitter are orthogonal to each other. However, this orthogonality is lost as the signals propagate through a multipath fading channel. A chip equalizer is employed in the W-CDMA receiver as a means to restore the orthogonality of the signal, and thereby improve the receiver performance.
Typically, chip equalizers are implemented as a Finite Impulse Response (FIR) filter. The chip equalizer tries to compensate for the multipath interference by inverting the channel. A known method for computing optimal chip equalizer filter coefficients using a direct inversion matrix method involves estimation of the matrix G from the expression G=HHH+βI, where HHH is the channel correlation matrix, I is identity matrix, and β is the scalar noise factor in a W-CDMA system.
The method for channel estimation to obtain the channel correlation matrix is usually straight forward, and can be estimated based on the common pilot signal in W-CDMA systems. However, to date there has been limited or no consideration of a optimal method of computing an estimate of the scalar noise factor in the W-CDMA system. A chip equalizer that only uses knowledge of channel estimation and does not take into account noise variance results in sub optimal performance compared to a receiver that takes both factors into account.